ABSD is a dimer of the selective aminopeptidase A (APA) inhibitor 3-amino 4-mercaptobutanesulfonic acid. ABSD has been proven to be an efficient anti-hypertensive agent, as described by Bodineau et al. in Hypertension 2008 51, 1318-1325. ABSD and use thereof as anti-hypertensive agent were disclosed in the patent application WO 2004/007441. The formula of ABSD is the following:

ABSD was first isolated under the form of the bis-hydrochloride of its sodium salt as described in WO 2004/007441. Said compound is not described to be crystalline and is clearly described as highly hygroscopic.
As described in the international patent application WO 2012/045849, ABSD was isolated also under its non-salified form as zwitterion. This zwitterion was found to form several hydrates. It typically exists as mixture of mono, di and tri-hydrate forms. The ratio of each hydrate form within ABSD zwitterion was found to be dependent of the storage conditions (temperature, atmospheric pressure and relative humidity (RH)). Indeed, the mixture of hydrated forms can evolve toward the trihydrate phase in less than two days when stored at circa 20° C. and RH>50%. The trihydrate phase appears to be the most stable phase under ambient conditions. Nevertheless, its dehydration more particularly toward the dihydrate form starts appearing at temperature below 30° C. This partial dehydration is often associated with a swelling effect, which may jeopardize the possibility to make tablets with that Active Principle Ingredient (also called herein API). Thus, the lack of thermal stability, the presence of water and the sensitivity to relative humidity in the zwitterionic form of ABSD could be a bar to its pharmaceutical formulations. Furthermore, processing and storage difficulties are likely to jeopardize the future development of ABSD zwitterionic form as viable API of pharmaceutical compositions.
Although therapeutic efficacy is the primary concern for a therapeutic agent, the salt and solid state forms (i.e., the crystalline forms and/or amorphous states) of a drug candidate can be critical to its pharmacological properties and to its development as a viable API. To prepare pharmaceutical compositions containing ABSD for administration to mammals, there is a need to produce this compound in a form having physical properties amenable to reliable formulation. Accordingly, there is a need in the art to provide improved forms of ABSD having enhanced properties, such as improved solubility or bioavailability and stability to heat, moisture, and/or light. Finding the most adequate form of the API for further drug development can reduce the time and the cost of that development.
In this context, the Applicant has now found that a particular crystalline phase with highly interesting properties can surprisingly be obtained by contacting ABSD with L-lysine, in particular in a 1:2 stoichiometry (ABSD:L-lysine).